Panama City Research Articles

Non-stationary reverberation observations from the shallow water TREX13 reverberation experiments using the FORA triplet array

A large experimental effort called TREX13 was conducted in April-May 2013 off Panama City, Florida. As part of this effort, reverberation and clutter measurements were taken in a fixed-fixed configuration in very shallow water (~20 m) over a 22 day period. Results are presented characterizing reverberation, clutter and noise in the 1800-5000 Hz band. The received data are taken from the triplet sub-aperture of the Five Octave Research Array (FORA). The array was fixed 2 m off the sea floor and data were passed to a nearby moored ship (the R/V Sharp). An ITC 2015 source transducer was fixed 1.1 m off the seafloor nearby. Pulses comprised of gated CWs and LFMs were used in this study. Matched filtered polar plots of the reverberation and clutter are presented using the FORA triplet beamformer. There are clear indications of biologic scattering. Some of the nearby shipwrecks are clearly visible in the clutter, as are reflections from a DRDC air-filled hose. The noise data show a surprising amount of time-dependent anisotropy. Some statistical characterization of these various components of the reverberation are presented using K-distribution based algorithms to note differences in the estimated shape parameter. Help from the Applied Physics Laboratory at the University of Washington was crucial to this effort. [Work supported by ONR code 322OA.]

Roughness measurement by laser profiler and acoustic scattering strength of a sandy bottom

The roughness of a sandy seabed off Panama City, FL, was measured with a laser profiler. This was the site of the target and reverberation experiment of 2013 (TREX13) in which propagation loss and reverberation strength were measured. The area may be characterized as having small scale roughness due to bioturbation overlaying larger sand ripples due to current activity. The area was largely composed of sand with shell hash crossed by ribbons of softer sediment at regular intervals. The roughness measurements were concentrated in the areas where the ribbons intersected the designated sound propagation track. Laser lines projected on the sand were imaged by a high-definition video recorder. The video images were processed to yield bottom profiles in three dimensions. Finally, the roughness data are used to estimate acoustic bottom scattering strength. [Work supported by the Office of Naval Research, Ocean Acoustics Program.]

Physics of backscattering in terms of mode coupling applied to measured seabed roughness spectra in shallow water

Energy conserving coupled integral equations for the forward and backward propagating modal components have been previously developed [J. Acoust. Soc. Am. 130, 2673–2680 (2011)]. A rough seabed surface leads to a backscattered field and modifies the interference structure of the forward propagating field. Perturbation theory applied to the basic coupled integral equations allows for physical insight into the correlation of the roughness spectrum to the forward and backward modal intensities and cross mode coherence. This study applies the Nx2D integral equation coupled-mode approach to 3-D roughness measurements and examines the physics of the coupling of the forward and backward field components and computes the modal intensities as a function of azimuth. The roughness measurements were made in about 20 m of water off Panama City, Florida. [Work supported by ONR Code 322 OA.]

A correlation analysis of the Naval Surface Warfare Center Panama City Division's (NSWC PCD) database of simulated and collected target scattering responses focused on automated target recognition

Recently, NSWC PCD participated in a number of computational and experimental efforts aimed at assembling a database of sonar scattering responses encompassing a variety of objects including UXO, cylindrical shapes, and other clutter-type objects. The range of data available on these objects consists of a simulated component generated with 3D finite element calculations coupled to a fast Helmholtz-equation-based propagation scheme, a well-controlled experimental component collected in NSWC PCD’s pond facilities, and a component of measurements in realistic underwater environments off Panama City, FL (TREX13 and BayEX14). The goal is to use the database to test schemes for automating reliable separation of these objects into desired classes. Here, we report on an initial correlation analysis of the database projected onto the target aspect vs frequency plane to assess the feasibility of the simulated component against the measured ones, to investigate some basic questions regarding environmental and range effects on class separation, and to try and identify phenomena in this plane useful for classification. [Work supported by ONR and SERDP.]

Overview of reverberation measurements in Target and Reverberation Experiment 2013

The Target and REverberation EXperiment 2013 (TREX13) was carried out off the coast of Panama City, Florida, from 22 April to 16 May, 2013. Two fixed-source/fixed-receiver acoustic systems were used to measure reverberation over time under diverse environmental conditions, allowing study of reverberation level (RL) dependence on bottom composition, sea surface conditions, and water column properties. Beamformed RL data are categorized to facilitate studies emphasizing (1) bottom reverberation; (2) sea surface impact; (3) biological impact; and (4) target echo. This presentation is an overview of RL over the entire experiment, summarizing major observations and providing a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis will be placed on the dependence of RL on local geoacoustic properties and sea surface conditions. [Work supported by ONR.]

Detection and classification of marine targets buried in the sediment using structural acoustic features

We present research on detection and classification of underwater targets buried in a saturated sediment using structural acoustic features. These efforts involve simulations using NRL’s STARS3D structural acoustics code and measurements in the NRL free-field and sediment pool facilities, off the coast of Duck, NC, and off the Coast of Panama City, FL. The measurements in the sediment pool demonstrated RVM classifiers trained using numerical data on two features—target strength correlation and elastic highlight image symmetry. Measurements off the coast of Duck were inconclusive owing to tropical storms resulting in a damaged projector. Extensive measurements were then carried out in 60 ft. of water in the Gulf using BOSS, an autonomous underwater vehicle with 40 receivers on its wings. The target field consisted of nine simulant-filled UXO and two false targets buried in the sediment and twenty proud targets. The AUV collected scattering data during north/south, east/west, and diagonal flights. We discuss the data analyzed so far from which we have extracted 3-D images and acoustic color constructs for 18 of the targets and demonstrated UXO/false target separation using a high dimensional acoustic color feature. Finally, we present related work involving targets buried in non-saturated elastic sediments. [This work is supported by ONR and SERDP.]

Persistence of sharp acoustic backscatter transitions observed in repeat 400 kHz multibeam echosounder surveys offshore Panama City, Florida, over 1 and 24 months

The Target and Reverberation Experiment 2013 (TREX13), conducted offshore Panama City, FL, from April to June 2013, sought to determine which environmental parameters contribute the most to acoustic reverberation and control sonar performance prediction modeling for acoustic frequencies between 1 kHz and 10 kHz. In that context, a multibeam echosounder operated at 400 kHz was used to map the seafloor relief and its high-frequency acoustic backscatter characteristics along the acoustic propagation path of the reverberation experiment. Repeat surveys were conducted a month apart, before and after the main reverberation experiment. In addition, repeat surveys were conducted at 200 kHz in April 2014. Similar mapping work was also conducted in April 2011 during a pilot experiment (GulfEx11) near the site chosen for TREX13. Both experiments revealed a persistent occurrence of sharp transitions from high to low acoustic backscatter at the bottom of swales. Hypotheses are presented for observable differences in bathymetry and acoustic backscatter in the overlap region between the GulfEx11 survey and the TREX13 surveys conducted 2 y apart. [Work supported by ONR 322 OA.]

Review of the Panama City Metro Project

The Metro rail transit system now under construction in Panama City, and planned extensions to that system, are poor investments for Panama. Depending on ridership, the US$1.88 billion construction cost of the first 13.7-kilometer line of the system could cost as much as $15 per rider. The costs of operating the line are likely to be greater than fare revenues, and maintenance costs for the system will grow until, after about 30 years, much of the infrastructure will need to be replaced at a probable cost of more than $1 billion.The government says Panama City needs a rail system because buses do not have the capacity to move the large numbers of people who enter and leave the center city each day. But the government has designed and is building a low-capacity rail system that will not be able to move more than about 6,400 people per hour. By comparison, transit buses can move more than 10,000 people per hour on city streets and double-decker buses can move at least 17,000 people per hour.The Panama City Metro will use three-car trains that the manufacturer says have a capacity of 600 people per train. But this is what transit experts call “crush capacity” in which everyone on board is pressed up against other people and/or car walls. Few people are willing to accept such crowding, so the actual capacity of each train is likely to be closer to about 375 people. At peak operation, the wait between trains will be about3.5 minutes, which means the system can move about 6,400 people per hour in each direction.A single train can hold more people than a transit bus, but buses can safely operate far more frequently than trains. Studies have found that a single bus stop can serve 42 buses per hour. If bus stops are staggered, with four stops every 400 feet, they can serve 168 buses per hour.Double-decker buses can easily hold more than 100 people, so they can move more 2.5 times as many people per hour as the Panama City Metro.The one thing rail transit does is create winners and losers. The winners include the companies that design and build the expensive rail lines, owners of property near rail stations, and the few people who will find it convenient to take a train from where most people don’t live to where most people don’t want to go. The losers include the taxpayers who have to support the train, owners of property away from the rail stations, and anyone who wants to travel to the many places the trains don’t go who suffers congestion and poor quality transportation because money that could have helped the entire city was spent on the rail line for an elite few.

Seismic and core investigation off Panama city, Florida, reveals sand ridge influence on formation of the shoreface ravinement

A high-resolution seismic reflection and vibracore survey was conducted on the inner shelf offshore of Panama City, Florida in April, 2011, to provide seabed characterization for Office of Naval Research acoustic reverberation experiments. The seafloor in this region is part of the MAFLA sand sheet: marine sands up to 5.5m thick extending from Mississippi to the Florida panhandle. The survey area spanned a transition in sand ridge morphology: northwest of the St. Andrews Bay inlet channel, sand ridges are up to ~4m tall, but die off southeast of the inlet channel. Analysis of reflection and core data documents a basal reflector beneath the ridges interpreted to be the shoreface ravinement separating estuarine and Pleistocene sediments below from the marine sands above scavenged from the eroded sediments. In the northwest, this surface truncates buried river channels filled with estuarine sediment. In the middle of the survey area, where sand ridges are ~2m high, the basal reflector is stratigraphically above the channel tops, and the presence of woody fragments in the intervening sand implies that it is relict barrier/estuarine material that has not been reworked into marine sand. In the southeast, where no sand ridges are present, the basal reflector is not observed, but the shoreface ravinement is identified in cores just ~0.4m below the seafloor; unreworked barrier/estuarine sands are up to ~4m thick above the channel tops. The stratigraphic progression implies reduced depth of shoreface erosion to the southeast, correlating with the reduction in sand ridge height. It is hypothesized here that sand ridge migration provides an addition to wave erosion in the formation of the shoreface ravinement along the lower shoreface and inner shelf, eroding through net negative bedform climb, and transferring sand to the sand ridges comprising the modern sand sheet.

Notaries of Color in Colonial Panama: Limpieza de Sangre,Legislation, and Imperial Practices in the Administration of the Spanish Empire

On July 20, 1740, King Philip V of Spain was given paperwork regarding a dispute over the adjudication of a notarial office in Panama City and, as usual, he was expected to make a decision. The king also had in hand recommendations from the Cámara of the Consejo de Indias. The king would have handled the case in a relatively straightforward manner, but for one fact—the two notaries involved in the public bid were of African descent.

Mycobacterium tuberculosis isolates from single outpatient clinic in Panama City exhibit wide genetic diversity.

Understanding Mycobacterium tuberculosis biodiversity and transmission is significant for tuberculosis control. This short report aimed to determine the genetic diversity of M. tuberculosis isolates from an outpatient clinic in Panama City. A total of 62 M. tuberculosis isolates were genotyped by 12 loci mycobacterial interspersed repetitive units-variable number of tandem repeats (MIRU-VNTR) and Spoligotyping. Forty-five (72.6%) of the isolates showed unique MIRU-VNTR genotypes, and 13 (21%) of the isolates were grouped into four clusters. Four isolates showed polyclonal MIRU-VNTR genotypes. The MIRU-VNTR Hunter-Gaston discriminatory index reached 0.988. The Spoligotyping analysis revealed 16 M. tuberculosis families, including Latin American-Mediterranean, Harlem, and Beijing. These findings suggest a wide genetic diversity of M. tuberculosis isolates at one outpatient clinic. A detailed molecular epidemiology survey is now warranted, especially following second massive immigration for local Panama Canal expansion activities.

Monaco with bananas, a tropical Manhattan, or a Singapore for Central America? Explaining rapid urban growth in Panama City, Panama

The built environment of Panama City, Panama, has undergone a transformative change over the past decade. Hundreds of high‐rise residential towers have sprung up in and around its central business district, eliciting comparisons with Singapore, New York and Dubai insofar as journalists, real estate boosters and politicians have associated the increase in tall buildings with a commensurate increase in global status. Concurrently, on the urban periphery, scores of uniform housing estates have been erected to house an upwardly mobile middle class. Triggered by the handover of the Panama Canal and the surrounding Canal Zone in 1999, the city's pronounced building boom has corresponded with the highest rates of economic growth in Latin America. This paper examines the complex factors behind the recent transformation of Panama City from a historical‐morphological perspective. While the drivers of demand for real property were primarily global, the determinants of supply have been highly localized, suggesting that the interface between the global and the local is a fundamental catalyst of changes in the urban landscape.

In the Limelight: Learning to Adapt

In the Limelight: Learning to Adapt

Estimates of the bottom reflection coefficient involving vector sensor

Estimates of the bottom reflection coefficient made in the frequency range of 4–8 kHz, as part of the Targets Reverberation Experiment (TREX) are presented. The TREX experiment took place in the Gulf of Mexico, near Panama City, FL, in waters 20 m deep. At the measurement site the sediments are loosely classified as fine sand. The reflection coefficient R is estimated over the nominal grazing angular range 1 to 20 deg., using measurements made at ranges 50 to 800 m and received on a vertical line array (length 1.6 m). The arrival time and magnitude of the bottom reflection is determined by the matched filtered output of a frequency modulated signal, 4–8 kHz. In addition, the match filter processing technique is applied to the vector sensor data (measured simultaneously and co-located with the line array.) This allows for an extraction of the active intensity contribution associated with the bottom reflection, and provides a vector intensity-based estimate of the bottom reflection coefficient. The estimates of the bottom reflection arrival time are also used to time-gate simultaneously transmitted cw tones (1–4 kHz) to analyze the Lloyd’s mirror pattern associated with seabed reflection. [Research supported by ONR, with partial support from ONRG.]

Observations of elliptical particle motion in shallow water and its dependence on source depth

Acoustic particle motion that follows an elliptical path coincides with a non-zero curl of the time-averaged intensity. This vector property is also observed as curvature in intensity (energy-flux) streamlines. Measurements of the acoustic intensity field made in shallow water are presented, along with simulations of the intensity field, to demonstrate some interesting relations between acoustic intensity and elliptical particle motion. Specifically, the direction in which intensity streamlines bend (sign of the curl of intensity) corresponds to the polarization of acoustic particle motion. For a source located in water, the polarization of particle motion depends on the modes of the underwater waveguide excited at a particular source depth. By raising a source up through the water column, an abrupt change in the polarization of particle motion can occur. This effect is examined with vector sensor data collected during an experiment near Panama City, FL. For a source located in air, elliptical particle motion is most evident a few wavelengths below the sea-surface where the contribution of the lateral (evanescent) wave is significant. This effect is examined with a recording of aircraft noise on both sides of the air-water interface made near Oak Harbor, WA.

Spatial variation of seabed acoustic bulk properties

The seabed is modeled as a poro-elastic medium with a rough interface. The spatial variation of bulk properties, along with the interface roughness, are important contributors to the acoustic scattering strength of the seabed. Their effects are often indistinguishable. While roughness may be measured directly, the variability in the bulk properties is more difficult to obtain. In a recent experiment over a sandy seabed off Panama City, FL, known as the target and reverberation experiment of 2013 (TREX13), the seabed roughness and the normal acoustic reflection loss were simultaneously measured using a laser profiler and a short range acoustic sounder deployed aboard a remotely operated vehicle (ROV). Using the measured roughness statistics, the fluctuations in acoustic reflection loss due to roughness were estimated. Subtracting the roughness contribution from the total measured reflection fluctuations, the component due to bulk property changes was estimated, from which the fluctuation in the bulk properties may be inverted. [Work supported by the Office of Naval Research, Ocean Acoustics Program.]

The influence of directional sea-surface waves on the acoustic intensity vector field

The effects of a rough sea surface on shallow water acoustic propagation are examined using experimental data collected from the Target and Reverberation Experiment (TREX) which took place off of the coast of Panama City, Florida in May 2013. During the experiment, the sea surface directional-wave spectrum was measured by a pair directional buoys moored at the experimental site. Acoustic measurements were collected using a bottom deployed recording tower (depth 20 m), that coherently recorded data from an accelerometer-based vector sensor, and a horizontal and vertical line array. Measurements using an active source, lowered from the stern of a research vessel, were made along propagation paths perpendicular and parallel to the surface wind-waves at source receiver ranges corresponding to approximately 10, 20, and 40 water depths. Results show that the directional properties of the rough sea-surface influence both the azimuthal and vertical distribution of the forward scattered intensity. A frequency dependence in vertical angular spreading is identified for the frequency range 1 to 3 kHz. A partial explanation for this effect originates from differences in the directional wave spectral level corresponding to forward scattering Bragg wavenumbers that are computed from the angles of the trapped modes.

Modeling the acoustic color of large aggregations of fish

The Naval Surface Warfare Center Panama City Division has developed a 3-D finite-element computer simulation system, PC-ACOLOR, for modeling the “acoustic color” (target strength as a function of frequency and aspect angle) of realistic elastic objects, either singly or in aggregations, that are near the bottom of the ocean or in deep water. It employs 3-D continuum mechanics throughout the entire computational domain. All objects and fluids are treated collectively as a single heterogeneous continuum; no engineering approximations are used anywhere. PC-ACOLOR was developed originally for modeling manmade structures, but it is intrinsic to finite-element modeling that the same code can be applied, unaltered, to any elastic objects, whether they be manmade structures or biological organisms. The talk will give an overview of PC-ACOLOR: structural acoustic concepts; modeling techniques; verification and validation; and examples of different types of objects that have been modeled, including aggregations of many fish.

Modeling the acoustic color of undersea structures

The Naval Surface Warfare Center Panama City Division has developed a 3-D finite-element computer simulation system, PC-ACOLOR, for modeling the “acoustic color” (target strength as a function of frequency and aspect angle) of single and multiple realistic elastic structures that are near to or straddling the water/sediment interface at the bottom of the ocean. Target strength is a measure of the intensity of scattered fields, but radiated noise can also be modeled, yielding sound pressure level as a function of frequency and aspect angle. PC-ACOLOR employs 3-D continuum mechanics throughout the entire computational domain; no engineering approximations are used anywhere. The talk will give an overview of PC-ACOLOR: important structural acoustic concepts, e.g., the relationship between 3-D modeling and evanescent wave solutions to the Helmholtz pde; modeling techniques; verification and validation; and examples of different types of vehicles and other structures that have been modeled.

Overview of the Target and Reverberation EXperiment 2013 (TREX13)

The Target and Reverberation Experiment (TREX13) is a shallow water reverberation experiment that endeavored to measure contemporaneously acoustic and adequate environmental data so detailed model/data comparison can be achieved and important environmental factors can be identified for different applications. TREX13 was sponsored by ONR Ocean Acoustics and the SERDP DoD programs. It was conducted during April to June of 2013 off the coast of Panama City, Florida, with participation from multiple institutions and involving three research vessels: The R/V Sharp and R/V Walton Smith from the United States, and the Canadian Force Auxiliary Vessel Quest. From a SONAR viewpoint, reverberation consists of two-way propagation and a single backscatter. Therefore, reverberation, transmission loss, and bottom backscatter were repeatedly measured over a time period of several weeks in the frequency band of 2–10 kHz, along with extensive environmental measurements. Discussed will be planning and execution of the field experiments, strategies and steps for data analysis, and modeling efforts.